It is necessary to insure that the breathing passageway of certain medical patients, e.g., those in surgery or intensive care, is kept open at all times. This is accomplished in the prior art by means of an endotracheal tube which is inserted through the patient's mouth or nose and extends through the patient's throat and into the patient's windpipe or trachea. These prior art tubes are hollow and open at both ends, and the end that extends outside the mouth or nose is anchored in place, usually with tape. Air can then pass through the tube into and out of the patient's lungs.
The principal drawback of the prior art tubes is that the distal end of the tube inside the patient must be inserted to and kept at a relatively specific position which is at about the midpoint of the trachea. This is because if the tube is inserted too far into the trachea, its distal end may extend into the bronchial tree for one lung, and thus the other lung will receive no air and may collapse. On the other hand, if the end of the tube is not inserted far enough, it may interfere with the vocal cords, or it may enter the esophagus, which opens near the bottom of the throat, and air would not reach the lungs. In a normal adult the trachea is about 11 centimeters in length, and the distal end of the tube is generally positioned at the trachea's approximate midpoint, and it may be anchored in place by expanding a balloon attached to the tube. This positioning, however, has much less margin for error in children or infants, whose tracheas are much shorter in length. Furthermore, for both adults and children, even if the tube is properly positoned initially, the movements of the patient often cause the tube to move up or down, and therefore the location of the distal end of the tube must be continuously monitored.
The prior art uses several methods for monitoring tube position. First, the tube position can be determined by x-ray, but notwithstanding the possible adverse effect of continued exposure to x-rays, the principal drawback here is that by the time the x-ray is taken, developed and returned, the tube may have moved again. Accordingly, two real-time monitoring methods are in wide use. They involve listening to the chest to hear is both lungs are filling and visual observation of the depth markings on the exposed tube. Neither of these real-time methods, however, is very precise.